1. The Howard Hughes Medical Institute and Departments of Biology, Chemistry and Biochemistry, Pharmacology, and Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, California, Department of Chemistry, The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, California, and the Science Department, San Diego Supercomputer Center, La Jolla, California